idpf: Don't hard code napi_struct size

The sizeof(struct napi_struct) can change. Don't hardcode the size to
400 bytes and instead use "sizeof(struct napi_struct)".

Suggested-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Signed-off-by: Joe Damato <jdamato@fastly.com>
Acked-by: Alexander Lobakin <aleksander.lobakin@intel.com>
Link: https://patch.msgid.link/20241004105407.73585-1-jdamato@fastly.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>